1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a hard disk drive using the magnetic recording technique, a method of manufacturing the magnetic recording medium, and a magnetic recording/reproducing apparatus.
2. Description of the Related Art
A magnetic storage device (HDD) mainly used in computers to record and reproduce information is recently gradually extending its applications because of its large capacity, inexpensiveness, high data access speed, data holding reliability, and the like. The HDD is now used in various fields such as household video decks, audio apparatuses, and car navigation systems. As the range of use of the HDD widens, demands for increasing the storage capacity or density of the HDD are also increasing. In recent years, high-density HDDs are being more and more extensively developed.
The longitudinal magnetic recording system is used in magnetic recording/reproducing apparatuses presently put on the market. In a magnetic recording layer used, magnetic grains for recording information have an easy axis of magnetization parallel to the substrate. The easy axis of magnetization is an axis in the direction of which magnetization easily points. In the case of a Co-based alloy, the c axis of the hcp structure of Co is the easy axis of magnetization. In a longitudinal magnetic recording medium, recording bits of a magnetic recording layer may become too small as the recording density increases. If this is the case, a so-called thermal decay effect by which information in these recording bits is thermally erased may worsen the recording/reproduction characteristics. Additionally, as the recording density increases, noise generated from the medium tends to increase due to the influence of an antimagnetic field generated in the boundary between the recording bits.
In contrast, in a so-called perpendicular magnetic recording system in which the easy axis of magnetization in the magnetic recording layer is oriented substantially perpendicularly to the substrate, the influence of an antimagnetic field between recording bits is small even when the recording density increases, and the operation is magnetostatically stable even at high density. Therefore, this perpendicular magnetic recording system is recently very noted as a technique which replaces the longitudinal recording system. The perpendicular magnetic recording medium is generally formed by a substrate, an orientation control underlayer for orienting a magnetic recording layer, a magnetic recording layer made of a hard magnetic material, and a protective layer for protecting the surface of the magnetic recording layer. In addition, a soft magnetic backing layer for concentrating a magnetic flux generated from a magnetic head during recording is formed between the substrate and underlayer.
Even in the perpendicular magnetic recording medium, to increase the recording density, it is necessary to reduce noise while the thermal stability is maintained. Various methods can be used to decrease the size of magnetic crystal grains for recording information, in order to increase the recording density. Generally, a method of decreasing the size of magnetic crystal grains in the recording layer is used. In the case of a CoCr-based magnetic layer presently extensively used, the grain size of magnetic grains is decreased by adding Ta or B to the layer or heating the layer at an appropriate temperature, thereby segregating nonmagnetic Cr in the grain boundary. However, downsizing of magnetic grains by Cr segregation has its limits. Also, the degree of this Cr segregation in the perpendicular magnetic recording medium is smaller than that in the longitudinal magnetic recording medium. Therefore, separation between the magnetic grains is insufficient, so the magnetic interaction between the grains remains relatively large. This poses the problem that transition noise between recording bits cannot be well reduced.
As a method of reducing this magnetic interaction, Jpn. Pat. Appln. KOKAI Publication No. 2001-76329 discloses a method of adding an oxide or nitride such as SiO2, ZrO2, or TiN to a recording layer, thereby forming a magnetic recording layer having a granular structure in which magnetic crystal grains are surrounded by this additive.
Unfortunately, the diffusion rate of an oxide or nitride is generally low, so precipitation to the magnetic crystal grain boundary is insufficient. Accordingly, a portion of the oxide or nitride which has not completely precipitated forms a supersaturated solid solution with the magnetic crystal grains, thereby disturbing the crystallinity and orientation of the magnetic crystal grains. Consequently, the signal-to-noise ratio (SNR) of the recording/reproduction (R/W) characteristics lowers.